I. Technical Field
The present invention relates to an apparatus for forming a plasma shutter configured to effectively block laser pulses returned as feedback light when generating radiations such as particle beams (proton, ion, etc.), X-rays, terahertz waves by irradiating a target with the laser pulses, and a method of the same.
II. Description of the Related Art
In recent years, advances have been made along diagnosis and medical treatment technology achieved by generating particle beams, such as proton beams or carbon-ion-beams, by irradiating a target with a laser beam, and then irradiating diseased parts of human bodies with the generated particle beams. The applicants of the invention have proposed a particle beam therapeutic device which achieves improved medical treatment and diagnosis with feedback from a radiologist reflected thereto in real time on the same floor in JP-A-2008-22994.
This particle beam therapeutic device is capable of exceedingly downsizing a particle beam generator to allow the same to be installed and used on the same floor as a diagnosis support devices (PET, X-ray CT and MRI, and ultrasonic therapy apparatus), and hence may be installed directly in a medical site and used for diagnoses and medical practices. FIG. 4 schematically shows an example of configuration of the particle beam generator in the particle beam therapeutic device.
A proton beam generating mechanism shown in FIG. 4 includes a laser device 21 configured to generate a laser beam 25 for obtaining a required proton beam, a laser beam transmitting device 22 configured to transmit the generated laser beam 25 via optical components such as a plurality of lenses and mirrors, a laser beam focusing part 23 configured to focus the transmitted laser beam 25 to a proton beam generating target, and a proton beam generating unit 24 configured to irradiate a target with the laser beam 25 focused by the laser beam focusing part 23 and generate a desired proton beam 26 from the target.
This example describes detailed configurations from the laser device 21 to the proton beam generating unit 24 which constitute the proton beam generating mechanism and, more specifically, a portion of the proton beam generating unit 24 which irradiates a thin tape target, which is the proton beam generating target, with the laser beam 25 to generate the proton beam 26 in detail.
The dimension of a focusing mirror 23a which constitutes the laser beam focusing part 23 is on the order of 200 mm, which is a dimension convenient for realizing an exceedingly compact proton beam generating mechanism. Specifically, in order to realize the downsizing, a thin film tape target 24a is employed in the proton beam generating unit 24, and the proton beam 26 is generated by irradiating the thin film tape target 24a as the proton beam generating target with the laser beam 25. The thin film tape target 24a, being wound around a tape supply reel 24b, is configured to be transferred by being taken up by a tape winding reel 24c, and a laser beam irradiation position is set by two rotating rolls 24d provided in abutment thereto so as to interpose the laser beam irradiation position between them.
In the configuration as described above, the laser device 21 generates the laser beam 25 having a required power and a beam diameter. The generated laser beam 25 is directed toward the thin film tape target 24a and focused on a tape target irradiation position P thereof through the laser beam transmitting device 22 and the laser beam focusing part 23. Irradiation of the laser beam 25 generates plasma. In this case, approximately 50% the laser beam 25 is absorbed by the plasma, and the remaining part of the laser beam 25 returns back to the upstream side of a laser system as feedback light.
In contrast, recently, development of proton beams generating laser for a compact laser-driven proton beam generating device for medical use is in progress. In order to do so, laser pulses having high peak intensity of energy are required. In order to obtain such laser pulses, a chirp pulse amplification (hereinafter referred to as CPA) technique is employed. FIG. 5 is a drawing schematically showing a configuration of a laser-driven proton beam generating device using the CPA technique in the related art. The CPA technique is also applicable to the device shown in FIG. 4.
In FIG. 5, reference numeral 1 designates a space having a pressure not higher than 1 Pa. Reference numeral 1′ designates a space having a pressure higher than 1 Pa and includes atmospheric pressure. Reference numeral 2 designates a proton beam generating target, which is a target for generating and accelerating the proton beam, reference numeral 3 designates a laser producing plasma, reference numeral 4 designates feedback light, reference numeral 5 designates a proton beam generating laser pulse, reference numeral 6 designates a focusing optics formed of a mirror or a lens, reference numeral 7 designates an optics formed of a polarizing plate or the like, reference numeral 8 designates a pulse compressor including a diffraction grating and a laser reflecting optics, and reference numeral 9 designates a laser-transmitting optics.
The proton beam generating laser pulse 5 emitted from a laser pulse generating device, not shown, enters a high-vacuum portion 1 from the laser-transmitting optics 9, passes through an optical system made up of the pulse compressor 8 including the diffraction grating and the laser reflecting optics, the optics 7, and the focusing optics 6, and is focused on the proton beam generating target 2. When the proton beam generating laser pulse 5 is focused on the proton beam generating target 2, the laser producing plasma 3 at a high density is generated. At this time, 50% the proton beam generating laser pulse 5 is absorbed by the laser producing plasma 3, and the remaining laser pulse returns back to the upstream of the laser system (from the pulse compressor including the diffraction grating and the laser reflecting optics to an oscillator) as the feedback light 4.
As described above, in the laser-driven particle beam generating device in the related art, proton and ion are generated and accelerated to be used in medical treatment and diagnosis. However, in the device in the related art of this type, the optics may be damaged due to the feedback light returned back to the upstream of the laser system when the plasma is generated. In particular, in the laser system employing CPA method, a problem of damages of the diffracting grating and the laser reflecting optics in the pulse compressor is serious.
Therefore, in the related art, methods using an optics, in which polarization of light such as Faraday isolator or Pockels cell is used, are employed as a countermeasure for the feedback light.
However, the optics such as the Faraday isolator or the Pockels cell are difficult to upsize. Therefore, it is difficult to install these optics immediately after the pulse compressor including the diffraction grating and the laser reflecting optics which enlarge the beam diameter of the laser pulse. It is difficult to install the Faraday isolator in terms of elongation of the pulse width when a damage threshold value and the CPA method are employed. In addition, since the Pockels cell is triggered by an electric signal, the required rise time is about 50 ps at the shortest. Therefore, it cannot block the feedback light satisfactorily.